Advanced gel sheet production

ABSTRACT

The present invention provides various methods for producing gel sheets in a continuous fashion. The embodiments of the present invention help reduce the time of producing gel sheets that is suitable for industrial manufacturing. Such gel sheets are used in manufacturing aerogel blankets used in a variety of applications including thermal and acoustic insulation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of, and incorporates byreference the entirety of pending U.S. patent application Ser. No.10/876,103, which was filed on Jun. 23, 2004, and which claimed priorityfrom, and incorporated by reference the entirety of U.S. ProvisionalPatent Application Ser. No. 60/482,359, which was filed on Jun. 24,2003, and which is now abandoned.

DESCRIPTION

1. Field of the Invention

This invention relates to the preparation of solvent filled gel sheetsin a continuous fashion. Such gel sheets are used in manufacturingaerogel blankets, aerogel composites, aerogel monoliths and otheraerogel based products.

2. Description of Related Art

Aerogels describe a class of material based upon their structure, namelylow density, open cell structures, large surface areas (often 900 m2/gor higher) and sub-nanometer scale pore sizes. Supercritical andsubcritical fluid extraction technologies are commonly used to extractthe fluid from the fragile cells of the material. A variety of differentaerogel compositions are known and may be inorganic or organic.Inorganic aerogels are generally based upon metal alkoxides and includematerials such as silica, carbides, and alumina. Organic aerogelsinclude, but are not limited to, urethane aerogels, resorcinolformaldehyde aerogels, and polyimide aerogels.

Low-density aerogel materials (0.01-0.3 g/cc) are widely considered tobe the best solid thermal insulators, better than the best rigid foamswith thermal conductivities of 10-15 mW/m-K and below at 100° F. andatmospheric pressure. Aerogels function as thermal insulators primarilyby minimizing conduction (low density, tortuous path for heat transferthrough the solid nanostructure), convection (very small pore sizesminimize convection), and radiation (IR absorbing or scattering dopantsare readily dispersed throughout the aerogel matrix). Depending on theformulation, they can function well at cryogenic temperatures to 550° C.and above. Aerogel materials also display many other interestingacoustic, optical, mechanical, and chemical properties that make themabundantly useful.

Low-density insulating materials have been developed to solve a numberof thermal isolation problems in applications in which the coreinsulation experiences significant compressive forces. For instance,polymeric materials have been compounded with hollow glass microspheresto create syntactic foams, which are typically very stiff, compressionresistant materials. Syntactic materials are well known as insulatorsfor underwater oil and gas pipelines and support equipment. Syntacticmaterials are relatively inflexible and of high thermal conductivityrelative to flexible aerogel composites (aerogel matrices reinforced byfiber). Aerogels can be formed from flexible gel precursors. Variousflexible layers, including flexible fiber-reinforced aerogels, can bereadily combined and shaped to give pre-forms that when mechanicallycompressed along one or more axes, give compressively strong bodiesalong any of those axes. Aerogel bodies that are compressed in thismanner exhibit much better thermal insulation values than syntacticfoams. Methods to produce these materials rapidly will facilitatelarge-scale use of these materials in underwater oil and gas pipelinesas external insulation.

Conventional methods for gel sheet and/or fiber-reinforced composite gelsheet production formed via sol-gel chemistry described in the patentand scientific literature invariably involve batch casting. Batchcasting is defined here as catalyzing one entire volume of sol to inducegelation simultaneously throughout that volume. Gel-forming techniquesare well-known to those trained in the art: examples include adjustingthe pH and/or temperature of a dilute metal oxide sol to a point wheregelation occurs (R. K. Iler, Colloid Chemistry of Silica and Silicates,1954, chapter 6; R. K. Iler, The Chemistry of Silica, 1979, chapter 5,C. J. Brinker and G. W. Scherer, Sol-Gel Science, 1990, chapters 2 and3).

U.S. Pat. No. 6,068,882 (Ryu) discloses an example of a fiber-reinforcedaerogel composite material that can be practiced with the embodiments ofthe present invention. The preferred aerogel composite precursormaterials used in the present invention are those like Cryogel®,Pyrogel®, or Spaceloft™ sold commercially by Aspen Aerogels,Incorporated. U.S. Pat. No. 5,306,555 (Ramamurthi et al.) discloses anaerogel matrix composite of a bulk aerogel with fibers dispersed withinthe bulk aerogel and a method for preparing the aerogel matrixcomposite.

SUMMARY OF THE INVENTION

This invention describes continuous and semi-continuous sol-gel castingmethods that are greatly improved over conventional batch sol-gelcasting methods for gel sheets, fiber-reinforced flexible gel sheets,and rolls of composite gel materials.

More specifically, the invention describes methods for continuouslycombining a low viscosity solution of a sol and an agent (heat catalystor chemical catalyst) that induces gel formation and forming a gel sheeton a moving element such as a conveyer belt with edges that defines thevolume of the formed gel sheet by dispensing the catalyzed sol at apredetermined rate effective to allow gelation to occur on the movingelement. The sol includes an inorganic, organic or a combination ofinorganic/organic hybrid materials. The inorganic materials includezirconia, yttria, hafnia, alumina, titania, ceria, and silica, magnesiumoxide, calcium oxide, magnesium fluoride, calcium fluoride, and anycombinations of the above. Organic materials include polyacrylates,polyolefins, polystyrenes, polyacrylonitriles, polyurethanes,polyimides, polyfurfural alcohol, phenol furfuryl alcohol, melamineformaldehydes, resorcinol formaldehydes, cresol formaldehyde, phenolformaldehyde, polyvinyl alcohol dialdehyde, polycyanurates,polyacrylamides, various epoxies, agar, agarose and any combinations ofthe above. Even more specifically, the methods describe the formation ofmonolithic gel sheets or fiber-reinforced gel composite having twoparts, namely reinforcing fibers and a gel matrix wherein thereinforcing fibers are in the form of a lofty fibrous structure (i.e.batting), preferably based upon either thermoplastic polyester or silicafibers, and more preferably in combination with individual randomlydistributed short fibers (microfibers) in a continuous orsemi-continuous fashion. The fibrous batting or mat material isintroduced onto the moving element for combination with the catalyzedsol prior to gelation.

Moreover, when a gel matrix is reinforced by a lofty batting material,particularly a continuous non-woven batting comprised of very low denierfibers, the resulting composite material when dried into an aerogel orxerogel product by solvent extraction, maintains similar thermalproperties to a monolithic aerogel or xerogel in a much stronger, moredurable form. The diameter of the fibers used is in the range of0.1-10,000 microns. In some cases the fiber batting, crimped fibers canbe distributed throughout the gel structure.

Even more specifically, the methods describe methods to continuously orsemi-continuously form gel composites by introduction of an energydissipation zone on the moving conveyor apparatus. The gelation of thecatalyzed sol can be enhanced by chemical or energy dissipation process.For instance, a controlled flux of electromagnetic (ultraviolet,visible, infrared, microwave), acoustic (ultrasound), or particleradiation can be introduced across the width of a moving sol volumecontained by a conveyor belt to induce sufficient cross-linking of thepolymers contained within the sol to achieve a gel point. The flux, thepoint and the area of radiation can be controlled along the conveyanceapparatus to achieve an optimized casting rate and desirable gelproperties by the time the terminus of the conveyor is reached for agiven section of gel. In this fashion, gel properties can be controlledin a novel fashion to a degree not possible with batch casting methods.In addition, another moving element rotating in the opposite directionto the first moving element can be used to provide the shape of the topportion of the gel sheets.

Still more specifically, a roll of gel composite material that isco-wound or corolled with a porous flow layer that facilitates solventextraction using supercritical fluids processing methods can be formedin a very small footprint using the method of the present invention.This is accomplished via infusing a predetermined amount of catalyzedsol in a rolled fiber-preform corolled with an impermeable spacer layer,gelling the infused roll, followed by un-rolling the gel compositearticle, removing the impermeable layer, and re-rolling of theincompletely cured body flexible gel composite with a porous spacerlayer. The method described in this invention provides great advantagein enhancing the rate of production of gel composite materials in assmall an area as possible.

Still more specifically, a method for producing gel sheets in acontinuous fashion is described in which gel sheets are produced by anyone of the above mentioned methods and are rolled into a plurality oflayers. This is a novel and effective way of producing gel sheets forefficient drying operations. In another feature, an optional spacermaterial is co-rolled with the gel sheets. Such a spacer material can bepermeable or impermeable in nature. Depending on the permeability of thespacer material, one can obtain a favorable flow pattern in a subsequentdrying. Spacer material also provides flow paths for subsequent silation(aging) fluids to easily pass through. In the drying they further helpby proving flow paths that effectively reduce the thickness of the gelsheet to be extracted in radial direction.

These and still further embodiments of the present invention aredescribed in greater detail below. The advantages of the methodsdescribed in this invention for processing monolith and fiber-reinforcedcomposite sheets in a continuous or semi-continuous fashion overpreviously described methods are many. For instance, the gel articlescan be fashioned continuously or semi-continuously provided allcomponents are fed into the apparatus at the appropriate rate. Thus,large volumes of material can be fashioned in a smaller production areathan with traditional batch casting requiring molds that must be filledand allowed to set for aging prior to solvent extraction to make aerogelor xerogel materials. Very long continuous sheets of fiber-reinforced,flexible gel material are readily fashioned using the methods of thisinvention because of the combined casting and rolling processing allowsa single molding surface to be continuously re-utilized within a smallproduction area. When rolls of gel are cast batchwise followed byroll-to-roll processing to place porous flow layers between layers ofgel material, the production footprint is diminished even further,increasing production capacity and potentially lowering production costsrelative to flat sheet batch casting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method of producing fiber reinforced gel sheetsusing a counter rotating conveyor belt.

FIG. 2 illustrates a method of producing fiber reinforced gel sheetsusing a single rotating conveyor belt.

FIG. 3 illustrates a method of producing fiber reinforced gel sheetsusing a counter rotating conveyor belt with additional cutting.

FIG. 4 illustrates a method of producing fiber reinforced gel sheetsusing a single rotating conveyor belt with additional cutting.

FIG. 5 illustrates the general flow diagram of catalyst-sol mixing priorto casting.

FIG. 6 illustrates an additional embodiment with dispensing thecatalyzed sol to a preformed roll including spacer layers.

FIG. 7 illustrates an additional embodiment for producing gel sheet byinducing a gelation zone.

FIG. 8 illustrates an additional embodiment for producing gel sheetswith one or more spacer layers.

DETAILED DESCRIPTION OF THE INVENTION

The invention(s) described herein are directed to producing solventfilled, nanostructured gel monolith and flexible blanket composite sheetmaterials. These materials give nanoporous aerogel bodies after allmobile phase solvents are extracted using a hypercritical solventextraction (supercritical fluid drying). For instance, the processesdescribed in this invention will offer significantly better productioncapacities for forming monolithic gel sheets or rolled gel compositearticles in a form factor that will facilitate removal of solvent in asubsequent supercritical fluids extraction procedure. The first methodoutlines a conveyor-based system that utilizes delivery of a lowviscosity, catalyzed sol mixture at one end and a system to cut andconvey formed monolithic (defined here as polymer or ceramic solidmatrix only, no fibers added) sheets of solvent filled gel material intoa system for further chemical treatment. The second method describes aconveyor-based system that utilizes delivery of a catalyzed sol mixtureof low viscosity at one end and a system to cut and conveysolvent-filled, fiber-reinforced gel composite sheets into a rollingsystem (with and without a porous separator flow layer) to produce aform factor ready for further treatment prior to supercritical fluidextraction. The third method describes a direct roll-to-roll transferprocess between two canisters in which the first hosts a direct “gel ina roll” reaction followed by unrolling and re-rolling the gel with aporous separator flow layer to prepare the form factor for furthertreatment prior to supercritical extraction. The three methods may beused in conjunction with controlled energy delivery methods tofacilitate the timing of gelation and the strength of the green bodiesformed. Energy in the form of ultrasound, heat, and various forms ofradiation can be used to induce gelation from a prepared sol mixture inaddition to classical methods of chemical catalysis (such as in a pHchange from a stable sol pH to one that facilitates gelation.

The matrix materials described in this invention are best derived fromsol-gel processing, preferably composed of polymers (inorganic, organic,or inorganic/organic hybrid) that define a structure with very smallpores (on the order of billionths of a meter). Fibrous materials addedprior to the point of polymer gelation reinforce the matrix materialsdescribed in this invention. The preferred fiber reinforcement ispreferably a lofty fibrous structure (batting or web), but may alsoinclude individual randomly oriented short microfibers, and woven ornon-woven fibers. More particularly, preferred fiber reinforcements arebased upon either organic (e.g. thermoplastic polyester, high strengthcarbon, aramid, high strength oriented polyethylene), low-temperatureinorganic (various metal oxide glasses such as E-glass), or refractory(e.g. silica, alumina, aluminum phosphate, aluminosilicate, etc.)fibers. The thickness or diameter of the fiber used in the embodimentsof the present invention is in the range of 0.1 to 10,000 micron, andpreferably in the range of 0.1 to 100 micron. In another preferredembodiment nanostructures fibers as small as 0.001 micron are used toreinforce the gel. Typical examples include carbon nanofibers and carbonnanotubes with diameters as small as 0.001 microns. Solvent filled gelsheets combining a ceramic solid (e.g. silica) and a mobile solventphase (e.g. ethanol) can be formed on a conveyor by continuous injectionof a catalyst phase into a sol phase and dispersing the catalyzedmixture onto a moving conveyor. Such materials will find use ininsulating transparencies, such as double-glazing windows in buildings.Because these gel materials are normally stiff and inflexible when theyare composed of a ceramic or cross-linked polymer matrix material withintercalated solvent (gel solvent) in the absence of fiberreinforcement, these materials need to be handled as molded if they arecontinuously cast. If the conveyor has molded edges that retain volume,then the gel can be directly cast onto the conveyor surface. If theconveyor has molds placed upon it, then the mold volumes can becontinuously filled with freshly catalyzed sol.

Suitable materials for forming inorganic aerogels are oxides of most ofthe metals that can form oxides, such as silicon, aluminum, titanium,zirconium, hafnium, yttrium, vanadium, and the like. Particularlypreferred are gels formed primarily from alcohol solutions of hydrolyzedsilicate esters due to their ready availability and low cost (alcogel).Organic aerogels can be made from polyacrylates, polystyrenes,polyacrylonitriles, polyurethanes, poly-imides, polyfurfural alcohol,phenol furfuryl alcohol, melamine formaldehydes, resorcinolformaldehydes, cresol formaldehyde, phenol formaldehyde, polyvinylalcohol dialdehyde, polycyanurates, polyacrylamides, various epoxies,agar, agarose, and the like (see for instance C. S. Ashley, C. J.Brinker and D. M. Smith, Journal of Non-Crystalline Solids, volume 285,2001).

In one preferred embodiment of the methods of this invention, energydissipation through a portion of the sol volume is utilized in aspecific location of a conveyor apparatus utilized for the gel casting.By controlling the area of the catalyzed sol that is exposed to heat orspecific flux of radiation (e.g. ultrasonic, x-ray, electron beam,ultraviolet, visible, infrared, microwave, gamma ray), a gelationphenomenon can be induced at a given point of a conveyor apparatus. Itis advantageous to control the timing of the gelation point with respectto the conveyor speed such that the material has adequate time to ageand strengthen prior to any mechanical manipulation at the terminus ofthe conveyor apparatus. Although the diffusion of polymer chains andsubsequent solid network growth are significantly slowed within theviscous gel structure after the gelation point, the maintenance of theoriginal gel liquid (mother liquor) for a period of time after gelationis essential to obtaining an aerogel that has the best thermal andmechanical properties. This period of time that the gel “ages” withoutdisturbance is called “syneresis”. Syneresis conditions (time,temperature, pH, solid concentration) are important to the aerogelproduct quality.

Gels are a class of materials formed by entraining a mobile interstitialsolvent phase within the pores of a solid structure. The solidstructures can be composed of inorganic, organic or inorganic/organichybrid polymer materials that develop a pore morphology in directrelation to the method of gelation, solvent-polymer interactions, rateof polymerization and cross-linking, solid content, catalyst content,temperature and a number of other factors. It is preferred that gelmaterials are formed from precursor materials, including variousfiber-reinforcement materials that lend flexibility to the formedcomposite, in a continuous or semi-continuous fashion in the form ofsheets or rolls of sheets such that the interstitial solvent phase canbe readily removed by supercritical fluids extraction to make an aerogelmaterial. By keeping the solvent phase above the critical pressure andtemperature during the entire, or at minimum the end of the solventextraction process, strong capillary forces generated by liquidevaporation from very small pores that cause shrinkage and pore collapseare not realized. Aerogels typically have low bulk densities (about 0.15g/cc or less, preferably about 0.03 to 0.3 g/cc), very high surfaceareas (generally from about 300 to 1,000 m2/g and higher, preferablyabout 700 to 1000 m2/g), high porosity (about 90% and greater,preferably greater than about 95%), and relatively large pore volume(about 3 mL/g, preferably about 3.5 mL/g and higher). The combination ofthese properties in an amorphous structure gives the lowest thermalconductivity values (9 to 16 mW/m-K at 37° C. and 1 atmosphere ofpressure) for any coherent solid material.

The monolithic and composite gel material casting methods described inthe present invention comprise three distinct phases. The first isblending all constituent components (solid precursor, dopants,additives) into a low-viscosity sol that can be dispensed in acontinuous fashion. The second involves dispensing the blended sol ontoa moving conveyor mold that may also have a synchronizedcounter-rotating top belt to form a molded upper surface. The secondphase may also include introduction of heat or radiation to the ungelledsol within a defined area of the moving conveyor apparatus to eitherinduce gelation or modify the properties of the gel such as gel modulus,tensile strength, or density. The third phase of the invention processinvolves gel cutting and conveyance of monolithic gel sheets to apost-processing area or co-rolling a flexible, fiber-reinforced gelcomposite with a flexible, porous flow layer to generate a particularlypreferred form factor of the material. The formed rolls of gel compositematerial and flow layer are particularly amenable to interstitialsolvent removal using supercritical processing methods. An example ofthe preferred gel casting method is shown in FIG. 1, which utilizes aconventional chemically catalyzed sol-gel process in combination with amoving conveyor apparatus with counter-rotating molding capability. Thefiber-reinforced, nanoporous gel composite can be mechanically rolled,with or without a porous flow layer, as shown in FIG. 1. FIG. 2 showsthe same process utilizing a moving conveyor belt with only a singlemolding surface (a continuously rotating bottom belt with molded sides).FIG. 3 shows how monolithic gel sheets, formed from a polymer sol(without added fiber reinforcing structures) can be formed continuouslyby deposition of a catalyzed sol solution onto a moving conveyor, andFIG. 4 illustrates the same procedure except a counter-rotating conveyormolding strategy is shown. The sols utilized in this invention are mixedand prepared, often by co-mixing with a chemical catalyst, prior todeposition onto the moving conveyor as shown in the block diagram ofFIG. 5. A related, but alternative embodiment of the invention processis shown in FIG. 6, in which a fiber and separator layer preform rollare infiltrated with a sol, and after initial gelation takes place,unrolled to separate the gel composite from the impermeable layer andsubsequently re-rolled with a permeable layer to prepare for furtherchemical processing.

The gel matrix of the preferred precursor materials for the presentinvention may be organic, inorganic, or a mixture thereof. Sols can becatalyzed to induce gelation by methods known to those trained in theart: examples include adjusting the pH and/or temperature of a dilutemetal oxide sol to a point where gelation occurs (The following areincorporated here by reference: R. K. Iler, Colloid Chemistry of Silicaand Silicates, 1954, chapter 6; R. K. Iler, The Chemistry of Silica,1979, chapter 5, C. J. Brinker and G. W. Scherer, Sol-Gel Science, 1990,chapters 2 and 3). Suitable materials for forming inorganic aerogels areoxides of most of the metals that can form oxides, such as silicon,aluminum, titanium, zirconium, hafnium, yttrium, vanadium, and the like.Particularly preferred are gels formed primarily from alcohol solutionsof hydrolyzed silicate esters due to their ready availability and lowcost (alcogel).

It is also well known to those trained in the art that organic aerogelscan be made from organic polymer materials including polyacrylates,polystyrenes, polyacrylonitriles, polyurethanes, polyamides, EPDM and/orpolybutadiene rubber solutions, polyimides, polyfurfural alcohol, phenolfurfuryl alcohol, melamine formaldehydes, resorcinol formaldehydes,cresol formaldehyde, phenol formaldehyde, polyvinyl alcohol dialdehyde,polycyanurates, polyacrylamides, various epoxies, agar, agarose, and thelike (see for instance C. S. Ashley, C. J. Brinker and D. M. Smith,Journal of Non-Crystalline Solids, volume 285, 2001).

Various forms of electromagnetic, acoustic, or particle radiationsources can be used to induce gelation of sol precursor materials on themoving conveyor apparatus. The literature contains a number of exampleswherein heat, ultrasonic energy, ultraviolet light, gamma radiation,electron beam radiation, and the like can be exposed to a sol materialto induce gelation. The use of energy dissipation (heat, acoustic,radiation) into a fixed zone of the conveyor apparatus, such that amoving sol pool interacts with a controlled energy flux for a fixedperiod of time is advantageous to control the properties of the gel aswell as the dried aerogel or xerogel material. This process isillustrated in FIG. 7.

Generally the principal synthetic route for the formation of aninorganic aerogel is the hydrolysis and condensation of an appropriatemetal alkoxide. The most suitable metal alkoxides are those having about1 to 6 carbon atoms, prefer-ably from 1-4 carbon atoms, in each alkylgroup. Specific examples of such compounds include tetraethoxysilane(TEOS), tetramethoxysilane (TMOS), tetra-n-propoxysilane, aluminumisopropoxide, aluminum sec-butoxide, cerium isopropoxide, hafniumtert-butoxide, magnesium aluminum isopropoxide, yttrium isopropoxide,titanium isopropoxide, zirconium isopropoxide, and the like. In the caseof silica precursors, these materials can be partially hydrolyzed andstabilized at low pH as polymers of polysilicic acid esters such aspolydiethoxysiloxane. These materials are commercially available inalcohol solution. Pre-polymerized silica precursors are especiallypreferred for the processing of gel materials described in thisinvention. Inducing gelation of metal oxide sols in alcohol solutions isreferred to as the alcogel process in this disclosure.

It is understood to those trained in the art that gel materials formedusing the sol-gel process can be derived from a wide variety of metaloxide or other polymer forming species. It is also well known that solscan be doped with solids (IR opacifiers, sintering retardants,microfibers) that influence the physical and mechanical properties ofthe gel product. Suitable amounts of such dopants generally range fromabout 1 to 40% by weight of the finished composite, preferably about 2to 30% using the casting methods of this invention.

Major variables in the inorganic aerogel formation process include thetype of alkoxide, solution pH, and alkoxide/alcohol/water ratio. Controlof the variables can permit control of the growth and aggregation of thematrix species throughout the transition from the “sol” state to the“gel” state. While properties of the resulting aerogels are stronglyaffected by the pH of the precursor solution and the molar ratio of thereactants, any pH and any molar ratio that permits the formation of gelsmay be used in the present invention.

Generally, the solvent will be a lower alcohol, i.e. an alcohol having 1to 6 carbon atoms, preferably 2 to 4, although other liquids can be usedas is known in the art. Examples of other useful liquids include but arenot limited to: ethyl acetate, ethyl acetoacetate, acetone,dichloromethane, and the like.

For convenience, the alcogel route of forming inorganic silica gels andcomposites are used below to illustrate how to create the precursorsutilized by the invention, though this is not intended to limit thepresent invention to any specific type of gel. The invention isapplicable to other gel compositions.

Alternatively, other sol preparation and gel induction methods can beutilized to make a precursor gel article using the processing methods ofthis invention, but the chemical approaches that allow for obtaining thelowest density and/or best thermally insulating articles are preferred.For example, a water soluble, basic metal oxide precursor can beneutralized by an aqueous acid in a continuous fashion, deposited onto amoving conveyor belt such as shown in FIGS. 1 and 2, and induced to makea hydrogel on the moving belt. Sodium silicate has been widely used as ahydrogel precursor. Salt by-products may be removed from the silicicacid precursor by ion-exchange and/or by washing subsequently formedgels with water after formation and mechanical manipulation of the gel.

After identification of the gel material to be prepared using themethods of this invention, a suitable metal alkoxide-alcohol solution isprepared. The preparation of aerogel-forming solutions is well known inthe art. See, for example, S. J. Teichner et al, Inorganic OxideAerogel, Advances in Colloid and Interface Science, Vol. 5, 1976, pp245-273, and L. D. LeMay, et al., Low-Density Microcellular Materials,MRS Bulletin, Vol. 15, 1990, p 19. For producing silica gel monolithsand fiber-reinforced containing silica gel composites useful in themanufacture of silica aerogel materials, typically preferred ingredientsare tetraethoxysilane (TEOS), water, and ethanol (EtOH). The preferredratio of TEOS to water is about 0.2-0.5:1, the preferred ratio of TEOSto EtOH is about 0.02-0.5:1, and the preferred pH is about 2 to 9. Thenatural pH of a solution of the ingredients is about 5. While any acidmay be used to obtain a lower pH solution, HCl, H2SO4 or HF arecurrently the preferred acids. To generate a higher pH, NH4OH is thepreferred base.

For the purposes of this patent, a lofty batting is defined as a fibrousmaterial that shows the properties of bulk and some resilience (with orwithout full bulk recovery). The preferred form is a soft web of thismaterial. The use of a lofty batting reinforcement material minimizesthe volume of unsupported aerogel while avoiding substantial degradationof the thermal performance of the aerogel. Batting preferably refers tolayers or sheets of a fibrous material, commonly used for lining quiltsor for stuffing or packaging or as a blanket of thermal insulation.

Batting materials that have some tensile strength are advantageous forintroduction to the conveyor casting system, but are not required. Loadtransfer mechanisms can be utilized in the process to introduce delicatebatting materials to the conveyor region prior to infiltration withprepared sol flow.

Suitable fibrous materials for forming both the lofty batting and thex-y oriented tensile strengthening layers include any fiber-formingmaterial. Particularly suitable materials include: fiberglass, quartz,polyester (PET), polyethylene, polypropylene, polybenzimidazole (PBI),polyphenylenebenzo-bisoxasole (PBO), polyetherether ketone (PEEK),polyarylate, polyacrylate, polytetrafluoroethylene (PTFE),poly-metaphenylene diamine (Nomex), polyparaphenylene terephthalamide(Kevlar), ultra high molecular weight polyethylene (UHMWPE) e.g.Spectra™, novoloid resins (Kynol), polyacrylonitrile (PAN), PAN/carbon,and carbon fibers.

FIG. 1 illustrates a method that produces fiber reinforced gel sheets ina continuous or semi-continuous fashion utilizing a sol dispensing andcatalyst mixing system and a counter-rotating conveyor belt moldapparatus. Gel composite sheets can be produced in rolled form ifmechanically wound at the end of the belt. The internal figure numberscorrespond as follows: 11 is a stable sol precursor solution, 12 is acatalyst to induce gelation of the sol when added in a proper quantityin controlled conditions, 13 indicates flow control positions, 14 is astatic mixer, 15 is the position in the fluid mixing system wherein thesol has been mixed thoroughly with catalyst, 16 is a scraper/lubricationdevice (optional), 17 is a fibrous batting material (may come indiscrete sheets or rolls that are fed into the assembly), 18 indicatestwo counter rotating belt assemblies that form molding surfaces alongthe length of which gelation occurs prior to the rolling assemblyindicated by 19.

FIG. 2 illustrates a method that produces fiber reinforced gel sheets ina continuous or semi-continuous fashion utilizing a sol dispensing andcatalyst mixing system and a single conveyor belt mold apparatus. Gelcomposite sheets can be produced in rolled form if mechanically wound atthe end of the belt. The internal figure numbers correspond as follows:21 is a stable sol precursor solution, 22 is a catalyst to inducegelation of the sol when added in a proper quantity in controlledconditions, 23 indicates flow control positions, 24 is a static mixer,25 is the position in the fluid mixing system wherein the sol has beenmixed thoroughly with catalyst, 26 is a scraper/lubrication device(optional), 27 is a fibrous batting material (may come in discretesheets or rolls that are fed into the assembly), 28 indicates a conveyorbelt assembly that forms a molding surface along the length of whichgelation occurs prior to the rolling assembly indicated by 29.

FIG. 3 illustrates a method that produces gel sheets in a continuous orsemi-continuous fashion utilizing a sol dispensing and catalyst mixingsystem and a counter-rotating conveyor belt mold apparatus. The internalfigure numbers correspond as follows: 30 is a stable sol precursorsolution, 31 is a catalyst to induce gelation of the sol when added in aproper quantity in controlled conditions, 32 indicates flow controlpositions, 33 is a static mixer, 34 and 35 are two counter rotating beltassemblies that form molding surfaces along the length of which gelationoccurs prior to the gel sheet cutting assembly indicated by 36. Discretegel sheets (37) are then ready for further processing.

FIG. 4 illustrates a method that produces gel sheets in a continuous orsemi-continuous fashion utilizing a sol dispensing and catalyst mixingsystem and a conveyor belt mold apparatus. The internal figure numberscorrespond as follows: 40 is a stable sol precursor solution, 41 is acatalyst to induce gelation of the sol when added in a proper quantityin controlled conditions, 42 indicates flow control positions, 43 is astatic mixer, 44 is a conveyor belt mold along the length of whichgelation occurs prior to the gel sheet cutting assembly indicated by 45.Discrete gel sheets (46) are then ready for further processing.

FIG. 5 illustrates the general flow diagram for mixing a sol and acatalyst in a mixing zone prior to casting (deposition) at a controlledrate onto a conveyor apparatus in a continuous fashion.

FIG. 6 illustrates an alternative casting method that involves a fiberand separator layer pre-form roll (60) in a container (61) beinginfiltrated with a sol (62), and after initial gelation takes place(63), unrolled (64) to separate the gel composite from the impermeablelayer (65) and subsequently re-rolled with a permeable layer (66) toform a gel composite/flow layer roll (67) in order to prepare forfurther chemical processing. Alternatively, Sol infiltrated pre-formroll can be directly dried with separator layer present in it andunrolled.

FIG. 7 illustrates a method that produces fiber reinforced gel sheets ina continuous or semi-continuous fashion utilizing a sol dispensingsystem and a single conveyor belt mold apparatus. Gelation is induced ina designed zone of the conveyor apparatus by exposure of the sol to heator radiation. The internal figure numbers correspond as follows: 70 is astable sol precursor solution, 71 is a catalyst to induce gelation ofthe sol when added in a proper quantity in controlled conditions, 72indicates flow control positions, 73 is a static mixer, 74 is theposition in the fluid mixing system wherein the sol has been mixedthoroughly with catalyst, 75 is a fibrous batting material (may come indiscrete sheets or rolls that are fed into the assembly), 76 is a devicethat dissipates energy into the sol or gel to alter its properties (e.g.inducing cross-linking), 77 indicates a conveyor belt assembly thatforms a molding surface along the length of which gelation occurs priorto the rolling assembly indicated by 78.

FIG. 8 illustrates another embodiment of the present invention, wheresol is dispensed onto a conveyer belt and allowed to gel as the conveyerbelt travels a specific distance (corresponding to a specified residencetime) and rolled onto a mandrel. While the gel sheet is rolled, apermeable spacer layer is co-rolled with the gel sheet such that any twolayers of the gel sheets are separated by the spacer layer. Optionallythis spacer could be impermeable. The rolled gel sheet assembly isfurther dried in a supercritical dryer. The spacer layer provideseffective flow paths during the supercritical extraction/drying. If theimpermeable spacer layer is used, it channels the flow of extractionfluid in axial direction. If the permeable spacer layer is used, anadditional radial flow pattern is also obtained. Depending on therequirements arising from the composition of the gel sheet, impermeableor permeable spacer layer is used to provide the necessary flow patternsin the supercritical extractor/dryer.

Further details and explanation of the present invention may be found inthe following specific examples, which describe the manufacture of themechanically densified aerogel composites in accordance with the presentinvention and test results generated there from. All parts and percentsare by weight unless otherwise specified.

EXAMPLE 1

Twenty gallons of silica sol produced by hydrolysis of a 20% TEOSsolution in ethanol (at pH 2 at room temperature for 24 hours) isintroduced into a stainless steel vessel equipped with a bottom drainconnected to fluid pump and flow meter. A separate container alsoequipped with a bottom drain, pump, and flow meter is filled with anexcess of ammoniated ethanol (1%). The two separate fluids are combinedat a fixed ratio using the flow meters through a static mixer anddeposited through a dispensing head onto a flat moving conveyor surface.The conveyor belt has flexible edges welded to the surface (38″ spacingis used in this example, but can be nearly any practical width), suchthat the dispensed sol is contained in volume. A pinch roller contactingthe front surface of the moving conveyor belt prevents back diffusion ofthe low viscosity sol. The belt speed is adjusted such that the gelationfront within the mixed sol (defined as the fixed position along theconveyor table at which the sol is no longer free flowing, taking on arubbery quality) appears halfway along the length of the table. A ratioof gelation time to syneresis time of 1:1 is preferred, but can varybetween 2:1 and 1:5 without problems. As the gelled sol reaches the endof the table, each silica gel plate is cut to size across the width andtransferred on a load-bearing plate into an alcohol bath for furtherprocessing.

EXAMPLE 2

Twenty gallons of silica sol produced by hydrolysis of a 20% TEOSsolution in ethanol (at pH 2 at room temperature for 24 hours) isintroduced into a stainless steel vessel equipped with a bottom drainconnected to fluid pump and flow meter. A separate container alsoequipped with a bottom drain, pump, and flow meter is filled with anexcess of ammoniated ethanol (1%). The two separate fluids are combinedat a fixed ratio using the flow meters through a static mixer anddeposited through a dispensing head onto a flat moving conveyor surface(38″ width between flexible edges). A roll of polyester batting (38inches wide) approximately 0.5″ thick is fed into the conveyor system atthe same linear speed as the belt. A pinch roller contacting the frontsurface of the moving conveyor belt prevents back diffusion of the lowviscosity sol, and another pinch roller in front of the sol depositionpoint is utilized to aid infiltration of the sol into the battingmaterial. The belt speed is adjusted such that the gelation front withinthe mixed sol (defined as the fixed position along the conveyor table atwhich the sol is no longer free flowing, taking on a rubbery quality)appears halfway along the length of the table. A ratio of gelation timeto syneresis time of 1:1 is preferred for flexible gel materials, butcan vary between 2:1 and 1:2 without problems. As the gelled sol reachesthe end of the table, the flexible gel composite is rolled onto acylindrical mandrel. A perforated polyethylene mesh is used to maintaintension of the roll as it is formed. The roll is then ready for furtherchemical processing and can be transferred using the mandrel as aload-bearing instrument.

EXAMPLE 3

Twenty gallons of silica sol produced by hydrolysis of a 20% TEOSsolution in ethanol (at pH 2 at room temperature for 24 hours) isintroduced into a stainless steel vessel equipped with a bottom drainconnected to fluid pump and flow meter. The silica sol is pumped at afixed rate through a dispensing head onto a flat moving conveyor surface(38″ width between flexible edges). A roll of polyester batting (38inches wide) approximately 0.5″ thick is fed into the conveyor system atthe same linear speed as the belt, prior to the sol deposition point. Apinch roller contacting the front surface of the moving conveyor beltprevents back diffusion of the low viscosity sol, and another pinchroller in front of the sol deposition point is utilized to aidinfiltration of the sol into the batting material. Arrays of ultrasoundtransducers coupled to the bottom of the belt through a lubricating gelare arranged at the midway point of the conveyor apparatus. The beltspeed and ultrasonic power and frequency are adjusted such that thegelation front within the mixed sol appears approximately halfway alongthe length of the table. As the gelled sol reaches the end of the table,the flexible gel composite is rolled onto a cylindrical mandrel. Aperforated polyethylene mesh is used to maintain tension of the roll asit is formed. The roll is then ready for further chemical processing andcan be transferred using the mandrel as a load-bearing instrument.

EXAMPLE 4

Twenty gallons of silica sol produced by hydrolysis of a 20%tetramethylorthosilicate (TMOS) solution in methanol (at pH 2 at roomtemperature for 4 hours) is introduced into a stainless steel vesselequipped with a bottom drain connected to fluid pump and flow meter. Aseparate container also equipped with a bottom drain, pump, and flowmeter is filled with an excess of ammoniated methanol (1%). The twoseparate fluids are combined at a fixed ratio using the flow metersthrough a static mixer and deposited through a dispensing head onto aflat moving conveyor surface. The silica sol is pumped at a fixed ratethrough a dispensing head onto a flat moving conveyor surface (38″ widthbetween flexible edges). A pinch roller contacting the front surface ofthe moving conveyor belt prevents back diffusion of the low viscositysol. The conveyor belt speed and sol deposition flow rate are matchedsuch that the gelation front for the (monolithic) silica gel sheetoccurs approximately half way along the length of the conveyor. The beltspeed is kept constant during the process to ensure that the ratio ofsyneresis time and gel time is approximately 1:1. As the aged silica gelsheet reaches a preferred length beyond the end of the conveyor belt (ona supporting surface to prevent cracking of the delicate gel structure),a cutting apparatus is engaged to separate the individual piece from thecontinuously moving gel. The new gel sheet is moved onto a load bearingplate and removed to another area for further treatment. This action isrepeated until all of the sol has been deposited on the table. Thisprocess can be run continuously as long as appropriately formulated solis replenished into the deposition apparatus.

EXAMPLE 5

Twenty gallons of silica sol produced by hydrolysis of a 20% TEOSsolution in ethanol (at pH 2 at room temperature for 24 hours) isintroduced into a stainless steel vessel equipped with a bottom drainconnected to fluid pump and flow meter. Ammoniated ethanol (1%) is addedwith stirring at a rate that maintains a near constant temperature untilthe pH of the sol reaches a value between 4 and 7. The pH adjusted(“catalyzed”) sol is deposited into a container through a roll ofpolyester batting (38 inches wide) approximately 0.5″ thick that hasbeen wound on a stainless steel mandrel with a polyethylene separatorlayer. The deposition is conducted in a fashion that prevents excessiveformation of air bubbles within the fiber volume, and can benefit fromthe use of resin transfer molding techniques or vacuum infiltrationtechniques known to those trained in the art. After gelation hasoccurred, the gel roll is unrolled prior to excessive stiffening (aratio of gelation time to syneresis time of greater than 1:1 ispreferred) wherein the impermeable plastic layer is removed and theflexible gel re-rolled with a permeable flow layer with appropriatetension into a separate canister (FIG. 6). The gelled roll is then readyfor further aging and chemical processing prior to supercritical drying.

In describing embodiments of the invention, specific terminology is usedfor the sake of clarity. For purposes of description, each specific termis intended to at least include all technical and functional equivalentsthat operate in a similar manner to accomplish a similar purpose.Additionally, in some instances where a particular embodiment of theinvention includes a plurality of system elements or method steps, thoseelements or steps may be replaced with a single element or step;likewise, a single element or step may be replaced with a plurality ofelements or steps that serve the same purpose. Moreover, while thisinvention has been shown and described with references to particularembodiments thereof, those skilled in the art will understand thatvarious other changes in form and details may be made therein withoutdeparting from the scope of the invention.

1. A method for casting gel sheets comprising the steps of: providing areinforcing layer; providing a separator layer; combining reinforcementlayer and separator layer to make a pre-form roll; infusing a sol intothe pre-form roll; and gelling the sol in the pre-form roll to producegel sheets.
 2. The method of claim 1 further comprising the step ofdrying said gel sheets using supercritical fluids.
 3. The method ofclaim 2 wherein drying is performed on the gelled pre-form roll.
 4. Themethod of claim 3 further comprising the step of unrolling the dried gelsheets.
 5. The method of claim 1, wherein the sol comprises a materialselected from the group consisting of zirconia, yttria, hafnia, alumina,titania, ceria, and silica, magnesium oxide, calcium oxide, magnesiumfluoride, calcium fluoride, and combinations thereof.
 6. The method ofclaim 1, wherein the sol comprises a material selected from the groupconsisting of polyacrylates, polyolefins, polystyrenes,polyacrylonitriles, polyurethanes, polyimides, polyfurfural alcohol,phenol furfuryl alcohol, melamine formaldehydes, resorcinolformaldehydes, cresol formaldehyde, phenol formaldehyde, polyvinylalcohol dialdehyde, polycyanurates, polyacrylamides, various epoxies,agar, and agarose, and combinations thereof.
 7. The method of claim 1wherein the gelling of the sol is enhanced by a process selected fromthe group consisting of (a) a chemical process, and (b) dissipating apredetermined quantity of energy from an energy source into across-sectional area of the sol.
 8. The method of claim 1 wherein theseparator layer is impermeable.
 9. The method of claim 1 wherein theseparator layer is a flexible sheet.
 10. The method of claim 1 whereininfusion of sol is performed using vacuum infiltration techniques.
 11. Amethod for preparing gel sheets, comprising the steps of: dispensing asol onto a moving element as a continuous sheet; rolling the dispensedsheet into a plurality of layers; and drying the layers.
 12. The methodof claim 11 wherein drying is accomplished using supercritical fluids.13. The method of claim 11 further comprising the step of providing afibrous batting material in the gel sheet.
 14. The method of claim 11further comprising the step of providing crimped fibers in the gelsheet.
 15. The method of claim 11, wherein the sol comprises a materialselected from the group consisting of zirconia, yttria, hafnia, alumina,titania, ceria, and silica, magnesium oxide, calcium oxide, magnesiumfluoride, calcium fluoride, and combinations thereof.
 16. The method ofclaim 11, wherein the sol comprises a material selected from the groupconsisting of polyacrylates, polyolefins, polystyrenes,polyacrylonitriles, polyurethanes, polyimides, polyfurfural alcohol,phenol furfuryl alcohol, melamine formaldehydes, resorcinolformaldehydes, cresol formaldehyde, phenol formaldehyde, polyvinylalcohol dialdehyde, polycyanurates, polyacrylamides, various epoxies,agar, and agarose, and combinations thereof.
 17. The method of claim 11wherein dispensed sol is gelled before rolling.
 18. The method of claim11 further comprising the step of introducing a spacer layer between anytwo gel sheet layers.
 19. The method of claim 18 wherein the spacerlayer is permeable.
 20. The method of claim 18 wherein the spacer layeris impermeable.
 21. The method of claim 17 wherein the gelling of thesol is enhanced by a process selected from the group consisting of (a) achemical process, and (b) dissipating a predetermined quantity of energyfrom an energy source into a cross-sectional area of the sol.
 22. Themethod of claim 13 wherein fibers in the fibrous batting have a diameterwithin a range of about 0.1 μm to about 10000 μm.
 23. The method ofclaim 13 wherein fibers in the fibrous batting have a diameter within arange of about 0.001 μm to about 10 μm.
 24. A process for casting gelsheets, comprising the steps of: providing a quantity of fibrous battingmaterial; introducing a impermeable layer to the fibrous battingmaterial and rolling into a fiber-roll preform having a plurality offibrous layers; infusing a quantity of sol into the fiber-roll preform;gelling the sol precursor in the fiber-roll preform; removing theimpermeable material to leave remaining a gel material; and introducinga quantity of a permeable material to separate the gel material into aplurality of layers wherein the permeable material has a form selectedfrom the group consisting of a mesh, a sheet, a perforated sheet, afoil, and a perforated foil.
 25. A process for casting gel sheets,comprising the steps of: providing a quantity of fibrous battingmaterial; rolling the fibrous batting material into a fiber-roll preformhaving a plurality of fibrous layers; infusing a quantity of sol intothe fiber-roll preform; gelling the sol in the fiber-roll preform; anddrying the gelled fiber-roll perform.